arch: arm: add per thread unique PAC key support

Add a config option to set unique PAC keys per thread and
make sure to retain them during context switch.

Signed-off-by: Sudan Landge <[email protected]>

Author: wearyzen

arch/arm/core/cortex_m/swap_helper.S

@@ -2,6 +2,7 @@
  * Copyright (c) 2013-2014 Wind River Systems, Inc.
  * Copyright (c) 2017-2019 Nordic Semiconductor ASA.
  * Copyright (c) 2020 Stephanos Ioannidis <[email protected]>
+ * Copyright 2025 Arm Limited and/or its affiliates <open-source-[email protected]>
  *
  * SPDX-License-Identifier: Apache-2.0
  */
@@ -313,6 +314,18 @@ SECTION_FUNC(TEXT, z_arm_pendsv)
 
 #endif
 
+#ifdef CONFIG_ARM_PAC_PER_THREAD
+    /* Read thread's dedicated PAC key and write them in the privileged PAC key registers.
+     */
+    add r0, r2, #_thread_offset_to_pac_keys
+    ldmia r0!, {r3-r6}
+    msr  PAC_KEY_P_0, r3
+    msr  PAC_KEY_P_1, r4
+    msr  PAC_KEY_P_2, r5
+    msr  PAC_KEY_P_3, r6
+    clrm {r3-r6}
+#endif /* CONFIG_ARM_PAC_PER_THREAD */
+
     /* load callee-saved + psp from thread */
     add r0, r2, #_thread_offset_to_callee_saved
     ldmia r0, {r4-r11, ip}

arch/arm/core/cortex_m/thread.c

@@ -1,7 +1,7 @@
 /*
  * Copyright (c) 2013-2014 Wind River Systems, Inc.
  * Copyright (c) 2021 Lexmark International, Inc.
- * Copyright (c) 2023 Arm Limited
+ * Copyright (c) 2023, 2025 Arm Limited and/or its affiliates <[email protected]>
  *
  * SPDX-License-Identifier: Apache-2.0
  */
@@ -20,6 +20,7 @@
 #include <zephyr/sys/barrier.h>
 #include <stdbool.h>
 #include <cmsis_core.h>
+#include <zephyr/random/random.h>
 
 #if (MPU_GUARD_ALIGN_AND_SIZE_FLOAT > MPU_GUARD_ALIGN_AND_SIZE)
 #define FP_GUARD_EXTRA_SIZE (MPU_GUARD_ALIGN_AND_SIZE_FLOAT - MPU_GUARD_ALIGN_AND_SIZE)
@@ -121,6 +122,12 @@ void arch_new_thread(struct k_thread *thread, k_thread_stack_t *stack, char *sta
 #if defined(CONFIG_USERSPACE)
 	thread->arch.priv_stack_start = 0;
 #endif
+#endif
+#ifdef CONFIG_ARM_PAC_PER_THREAD
+	/* Generate PAC key and save it in thread context to be set later
+	 * when the thread is actually switched in
+	 */
+	sys_csrand_get(&thread->arch.pac_keys, sizeof(struct pac_keys));
 #endif
 	/*
 	 * initial values in all other registers/thread entries are
@@ -541,6 +548,9 @@ void arch_switch_to_main_thread(struct k_thread *main_thread, char *stack_ptr,
 #endif
 #endif /* CONFIG_BUILTIN_STACK_GUARD */
 
+#ifdef CONFIG_ARM_PAC_PER_THREAD
+	__set_PAC_KEY_P((uint32_t *)&main_thread->arch.pac_keys);
+#endif
 	/*
 	 * Set PSP to the highest address of the main stack
 	 * before enabling interrupts and jumping to main.

arch/arm/core/offsets/offsets_aarch32.c

@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2013-2014 Wind River Systems, Inc.
+ * Copyright 2025 Arm Limited and/or its affiliates <[email protected]>
  *
  * SPDX-License-Identifier: Apache-2.0
  */
@@ -32,6 +33,10 @@
 GEN_OFFSET_SYM(_thread_arch_t, basepri);
 GEN_OFFSET_SYM(_thread_arch_t, swap_return_value);
 
+#if defined(CONFIG_ARM_PAC_PER_THREAD)
+GEN_OFFSET_SYM(_thread_arch_t, pac_keys);
+#endif
+
 #if defined(CONFIG_CPU_AARCH32_CORTEX_A) || defined(CONFIG_CPU_AARCH32_CORTEX_R)
 GEN_OFFSET_SYM(_thread_arch_t, exception_depth);
 GEN_OFFSET_SYM(_cpu_arch_t, exc_depth);

arch/arm/include/offsets_short_arch.h

@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2019 Carlo Caione <[email protected]>
+ * Copyright 2025 Arm Limited and/or its affiliates <[email protected]>
  *
  * SPDX-License-Identifier: Apache-2.0
  */
@@ -59,6 +60,10 @@
 	(___thread_stack_info_t_start_OFFSET + ___thread_t_stack_info_OFFSET)
 #endif
 
+#if defined(CONFIG_ARM_PAC_PER_THREAD)
+#define _thread_offset_to_pac_keys \
+	(___thread_t_arch_OFFSET + ___thread_arch_t_pac_keys_OFFSET)
+#endif
 
 /* end - threads */
 

arch/common/Kconfig

@@ -76,6 +76,23 @@ config ARM_MPU
 	  of full partitioning the default behavior for the ARMv8-M and ARMv8-R MPU
 	  driver.
 
+config ARM_PAC_PER_THREAD
+	bool "Set cryptographically secure PAC key per thread"
+	depends on ARM_PAC
+	depends on ENTROPY_DEVICE_RANDOM_GENERATOR || TIMER_RANDOM_GENERATOR
+	depends on !USERSPACE
+	help
+	  Select this option to generate and use unique keys per thread to generate Pointer
+	  Authentication Code.
+	  Internally, sys_csrand_get() is used as part of arch_new_thread() to generate
+	  cryptographically secure random keys, which are saved in the thread's
+	  architecture-specific context. These keys are then loaded into the PAC key registers
+	  before switching to the thread during context switches.
+	  Applications can chose to have hardware based random keys generator by selecting the right
+	  generator from RNG_GENERATOR_CHOICE or by selecting TIMER_RANDOM_GENERATOR for testing
+	  with pseudo random keys.
+	  Note: GCC version 14.3 or higher is needed to support this option.
+
 config ARM_PAC
 	bool
 	help

doc/hardware/arch/arm_cortex_m.rst

@@ -459,6 +459,11 @@ automatically selected based on the branch protection option chosen for
 :kconfig:option:`CONFIG_ARM_PACBTI`. These configuration options enforce PACBTI by enabling
 corresponding PACBTI bits in CONTROL register and in the FVP.
 
+To further enhance pointer authentication, Zephyr supports using cryptographically secure,
+per-thread PAC keys by enabling :kconfig:option:`CONFIG_ARM_PAC_PER_THREAD`.
+For more details on key generation sources and configuration, refer to the Kconfig help for
+:kconfig:option:`CONFIG_ARM_PAC_PER_THREAD`.
+
 **Limitations:**
 
 - Only builds targeting Armv8.1-M Mainline processors with PACBTI hardware support (e.g.,

include/zephyr/arch/arm/thread.h

@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2017 Intel Corporation
+ * Copyright 2025 Arm Limited and/or its affiliates <[email protected]>
  *
  * SPDX-License-Identifier: Apache-2.0
  */
@@ -60,6 +61,15 @@ struct _preempt_float {
 };
 #endif
 
+#if defined(CONFIG_ARM_PAC_PER_THREAD)
+struct pac_keys {
+	uint32_t key_0;
+	uint32_t key_1;
+	uint32_t key_2;
+	uint32_t key_3;
+};
+#endif
+
 struct _thread_arch {
 
 	/* interrupt locking key */
@@ -134,6 +144,10 @@ struct _thread_arch {
 #endif
 #endif
 #endif
+
+#if defined(CONFIG_ARM_PAC_PER_THREAD)
+	struct pac_keys pac_keys;
+#endif
 };
 
 #if defined(CONFIG_FPU_SHARING) && defined(CONFIG_MPU_STACK_GUARD)

modules/cmsis_6/cmsis_core_m_defaults.h

@@ -147,4 +147,10 @@ typedef enum {
 #error "Unknown Cortex-M device"
 #endif
 
+#ifdef CONFIG_ARM_PAC
+/* This provides apis to set/get PAC keys */
+#define __ARM_FEATURE_PAUTH 1
+#include <m-profile/armv81m_pac.h>
+#endif
+
 #endif /* ZEPHYR_MODULES_CMSIS_6_CMSIS_CORE_M_DEFAULTS_H_ */